1. Field of the Invention
The present invention relates to a multi-layer printed circuit board (PCB) and more particularly to a multi-layer PCB for a keyboard.
2. Description of Related Art
A keyboard includes a base having an upper opening, a top cover, a printed circuit board (PCB), and multiple keys. The multiple keys are mounted on the top cover and the top cover is mounted over the upper opening of the base. The PCB is mounted in the base and has multiple thin film switches corresponding to the multiple keys on the top cover. Therefore, when one key is pressed, the pressed key will touch the corresponding thin film switch on the PCB, and then the PCB will output an electronic signal to a computer connected to the keyboard.
With reference to FIG. 8, a conventional dual-layer PCB of the keyboard has an upper board (10), an insulating separation layer (30) and a lower board (20). The upper board (10) and the lower board (20) have multiple circuit lines (12, 13) (22, 23). The insulating separation layer (30) is positioned between the upper and lower boards (10, 20) and has multiple through holes (31).
Multiple thin film switches are mounted between the upper board (10) and the lower board (20), but only one thin film switch is shown in FIG. 8. Each thin film switch includes two thin conductive elements (50, 60) respectively formed on the upper and lower boards (10, 20). The two thin conductive elements (50, 60) are aligned with one of the through holes (31), and each is formed as a metal sheet. The two thin conductive elements (50, 60) are respectively connected to the circuit lines (12, 13)(22, 23).
The two thin conductive elements (50, 60) are separated by the insulting insulating separation layer (30) and aligned with the corresponding through hole (31). When one key on the top cover of the keyboard is pressed, the two thin conductive elements (50, 60) corresponding to the pressed key touch with each other and are connected electrically through the through hole (31) in the insulating separation layer (30). When the force applied to the key is released, the two thin conductive elements (50, 60) are disconnected electrically. Therefore, a thickness of the insulating separation layer (30) determines the sensitivity of touching a key. Many types of keyboards having different sensitivity of touch are produced, since the insulating separations with different thickness are used in the PCBs of the keyboards. Accordingly, the factory for fabricating the multi-layer PCB has to prepare many different insulating separation layers to fabricate the keyboards having different sensitivity of touch.
Further, since the thin conductive elements (50, 60) are formed on the upper and lower boards (10, 20), exposure surfaces of the thin conductive elements (50, 60) are easily shrunk to form to an uneven face in a curing procedure of fabricating the upper and lower boards (10, 20). Thus, the uneven faces decrease a touching area of the thin conductive elements (50, 60) to make the sensitivity of the thin film switch poor.
Therefore, the present invention provides a multi-layer PCB for a keyboard to overcome the drawback of the conventional multi-layer PCB.